He also learned his former mother-in-law, Darlene Phillips, 63, had been charged with murder in his son's death.
Dubbed "Wylie's Angel," police later identified the little boy who had been left for dead at in a field as Gerren Joseph Isgrigg, age 6.
In a phone interview, Isgrigg said he felt he had been cut out of his son's life after he divorced his wife, Nyki Phillips, in 2006. Isgrigg has seen Gerren only twice since 2005. He said he thought Gerren was in the care of his former wife.
"I had no idea she had passed him off to her mom and dad," Isgrigg said.
Isgrigg said his son was born one month premature and shortly after was diagnosed with West Syndrome, a disorder that causes siezures.
He said he wasn't surprised police found his son weighing only 28 pounds. He said there was a very short time in Gerren's life when he drank from a bottle and knew how to swallow, but when he stopped doing that he was fed through a feeding tube.
Isgrigg said he was shocked and angry at what transpired.
"I don't understand how if a child is going downhill, you don't look into it, you don't go to a doctor. There is no excuse for that," he said.
His former mother-in-law is being held in a Collin County jail on a $500,000 bond.
Autopsy results are still pending.